The present invention relates to processes and compositions for forming diffusion coatings. More particularly, this invention relates to a slurry coating composition and process for forming a diffusion aluminide coating on a substrate surface.
The hot gas path within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc. Aluminum-containing coatings, particularly diffusion aluminide coatings, have found widespread use as environmental coatings on gas turbine engine components. During high temperature exposure in air, aluminum-containing coatings form a protective aluminum oxide (alumina) scale or layer that inhibits corrosion and oxidation of the coating and the underlying substrate.
Diffusion coatings can be generally characterized as having an additive layer that primarily overlies the original surface of the coated substrate, and a diffusion zone below the original surface. The additive layer of a diffusion aluminide coating contains the environmentally-resistant intermetallic phase MAI, where M is iron, nickel or cobalt, depending on the substrate material (mainly β(NiAl) if the substrate is Ni-base). The diffusion zone comprises various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate.
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, often require some form of thermal protection in addition to an environmental coating. One approach is to deposit a ceramic thermal barrier coating (TBC) on the external surfaces of the component. Another approach is to configure the component to provide cooling air flow through internal passages within the component. For more demanding applications, it can be necessary to utilize internal cooling in combination with a TBC. Temperatures inside internal cooling passages can be sufficiently high to require a diffusion aluminide coating for oxidation protection. Because the size and geometry of the internal passages and cooling holes of air-cooled components are critical to maintaining the required amount of coolant flow, processes by which diffusion aluminide coatings are deposited on the external and internal surfaces of air-cooled components should be capable of producing coatings of uniform thickness and leave minimal residue that would negatively affect the cooling flow through the component.
Diffusion aluminide coatings are generally formed by depositing and diffusing aluminum into the surface of a component at temperatures at or above about 1400° F. (about 760° C.). Notable processes include pack cementation and vapor phase aluminizing (VPA) techniques, and diffusing aluminum deposited by chemical vapor deposition (CVD), slurry coating, or another deposition process. Pack cementation and VPA processes generally involve the use of an activator to transport aluminum from an aluminum source to the surface of the component being coated. For example, a halide activator (typically ammonium halide or an alkali metal halide) can be reacted with an aluminum-containing source (donor) material to form an aluminum halide gas (such as aluminum fluoride (AlF3) or aluminum chloride (AlCl3)) that travels to the surface of the component, where it reacts to reform and deposit aluminum. In contrast, aluminum deposited by slurry coating is typically diffused without an activator, relying instead on melting and subsequent diffusion of the deposited aluminum.
The processing temperature and whether an activator is used will influence whether a diffusion coating is categorized as an outward-type or inward-type. Outward-type coatings are formed as a result of using higher temperatures (e.g., at or near the solution temperature of the alloy being coated) and lower amounts of activator as compared to inward-type coatings. In the case of a nickel-based substrate, such conditions promote the outward diffusion of nickel from the substrate into the deposited aluminum layer to form the additive layer, and also reduce the inward diffusion of aluminum from the deposited aluminum layer into the substrate, resulting in a relatively thick additive layer above the original surface of the substrate. Conversely, lower processing temperatures and larger amounts of activator reduce the outward diffusion of nickel from the substrate into the deposited aluminum layer and promote the inward diffusion of aluminum from the deposited aluminum layer into the substrate, yielding an inward-type diffusion coating characterized by an additive layer that extends below the original surface of the substrate. The choice of donor material influences whether an outward or inward-type diffusion coating can be produced, since aluminum alloys such as CrAl, CoAl, FeAl, TiAl, etc., have higher melting temperatures than unalloyed aluminum and therefore can be used with the higher processing temperatures used to form outward-type coatings. Though both outward and inward-type diffusion aluminide coatings are successfully used, outward-type diffusion aluminide coatings typically have a more ductile and stable nickel aluminide intermetallic phase and exhibit better oxidation and low cycle fatigue (LCF) properties as compared to inward-type diffusion aluminide coatings.
Pack cementation and VPA processes are widely used to form aluminide coatings because of their ability to form coatings of uniform thickness. In pack cementation processes, the aluminum halide gas is produced by heating a powder mixture comprising the source material, the activator, and an inert filler such as calcined alumina. The ingredients of the powder mixture are mixed and then packed and pressed around the component to be treated, after which the component and powder mixture are heated to a temperature sufficient to vaporize the activator. The vaporized activator reacts with the source material to form the volatile aluminum halide, which then reacts at the component surface to form a aluminide coating, typically a brittle inward-type coating with high aluminum content due to the use of a relatively low treatment temperature to minimize sintering of the pack material and high activity required of the activator to offset the dilution effect of the inert filler. In contrast, VPA processes are carried out with the source material placed out of contact with the surface to be aluminized. Depending on the processing temperature and amount of activator used, VPA coatings can be inward or outward-type.
A disadvantage of performing a pack cementation process on an air-cooled component is that particles of the source material and inert filler can sinter and become trapped in the cooling passages and holes, requiring labor-intensive removal of the particles so as not to negatively affect cooling flow through the component. On the other hand, a difficulty encountered with VPA processes is the inability to produce a uniform aluminide coating on all internal passages of a component.
Slurries used to form diffusion aluminide coatings are typically aluminum rich, containing only an unalloyed aluminum powder in an inorganic binder. The slurry is directly applied to surfaces to be aluminized, and aluminizing occurs as a result of heating the component in a non-oxidizing atmosphere or vacuum to a temperature above 1400° F. (about 760° C.), which is maintained for a duration sufficient to melt the aluminum powder and diffuse the molten aluminum into the surface. The thickness of a diffusion aluminide coating produced by a slurry method is typically proportional to the amount of the slurry applied to the surface, and as such the amount of slurry applied must be very carefully controlled. The difficulty of consistently producing diffusion aluminide coatings of uniform thickness has discouraged the use of slurry processes on components that require a very uniform diffusion coating and/or have complicated geometries, such as air-cooled turbine blades. As a result, though capable of forming diffusion aluminide coatings on internal and external surfaces, slurry coating processes have been typically employed to coat limited, noncritical regions of gas turbine engine components. Another limitation of slurry coating processes is that, because of the use of unalloyed aluminum, they are typically performed at relatively low temperatures (e.g., below 1800° F./980° C.), and are therefore limited to producing an inward-type coating with high aluminum content.
U.S. Pat. No. 6,444,054 to Kircher et al. discloses an alternative slurry composition that contains an activator and produces an inward-type diffusion aluminide coating. The slurry composition contains a chromium-aluminum (Cr—Al) alloy powder, lithium fluoride (LiF) as the activator, and an organic binder such as hydroxypropylcellulose dissolved in a solvent. As with pack cementation and VPA processes, the LiF activator vaporizes and reacts with the aluminum in the alloy powder to form a volatile aluminum halide, which then reacts at the component surface to form an aluminide coating. According to Kircher et al., the applied slurry coating is heated to a temperature of about 1600° F. to about 2000° F. (about 870° C. to about 1090° C.) to form the inward-type diffusion aluminide coating. The ability to form inward-type coatings over such a wide range of temperatures appears to be the result of the particular activator used, LiF, being highly reactive. The coating is said to have a uniform thickness that is largely independent of the as-applied slurry coating thickness. However, slurry thicknesses of greater than 0.050 inch (about 1.3 mm) are not attempted. Finally, Kircher et al.'s slurry leaves residues that are said to be removed by wire brush, glass bead or oxide grit burnishing, high pressure water jet, or other conventional methods, suggesting that the residues are firmly attached to the coating surface. As such, it appears that Kircher et al.'s slurry is not suitable for use on internal surfaces that cannot be reached by such surface treatments.
There is an ongoing need for coating processes that are capable of depositing diffusion aluminide coatings of uniform thickness on internal and external surfaces, and that do not entail labor-intensive cleaning to remove coating residue remaining at the completion of the coating process. It would be particularly desirable if such a process was capable of being performed over a wide range of temperatures and capable of forming both inward and outward-type diffusion aluminide coatings.